I have a ticket on Thalys getting off at Rotterdam. Now my plans have changed and I would like to stay on the train until Schiphol. I tried to communicate via email with no luck. I also looked at the Dutch train website and I believe it is possible to buy the additional route however I do not have a Dutch bank account therefore it would not be possible to book online. I would appreciate anyone who has any suggestions for me.
Ask the conductor of your train whether this is possible (do so as soon as you board) to buy an add-on. If that doesn't work, get off in Rotterdam and buy a ticket on a domestic high-speed train (Intercity Direct) to Schiphol. There are trains on that route every 30 minutes.
Are you starting from Paris? If you can't get any help in advance, I'd check in person at the train station there to see if you can extend your ticket to Schiphol. I'd do it in advance of the journey so that everything can get worked out without a lot of stress. If you don't want to bother with Thalys and just want to take the cheaper IC from Rotterdam to Schiphol, you can buy domestic Dutch train tickets online at the Belgian Railway website: www.b-europe.com. It's the same price as if you bought it on the Dutch site.
If your Thalys ticket is an economy, nonrefundable, non-changeable fare, then they will not apply that cost toward a new, longer ticket. You'll simply buy a new ticket for the remaining distance on Thalys (cheaper if you book ahead) or on a local service if you change trains. If the fare you purchased does allow an exchange, then do that soon through the original vendor.
Thanks for all the replies.